Lightweight structural articles



Apri28,1970 M. BVORDAHL 3,508,599

y LIGHTWEIGHT STRUCTURAL ARTICLES Original Filed June 5. 1961 2Sheets-Sheet 1 Milton B.Vordohl ATTORNEY April 28, 1970 M. B. voRDAHL3,508,599

LIGHTWEIGHT STRUCTURAL ARTICLES Original Filed June 5. 1961 2Sheets-Sheet 2 INVENToR Milton B. Vordohl ATTORNEY Figs United StatesPatent Office -3,508,599 Patented Apr. 28, 1970 3,508,599 LIGHTWEIGHTSTRUCTURAL ARTICLES Milton B. Vordahl, Beaver, Pa., assigner to CrucibleSteel Company of America, Pittsburgh, Pa., a corporation of New `lerseyOriginal application June 5, 1961, Ser. No. 114,903.

Divided and this application Sept. 23, 1964, Ser.

Int. Cl. B22d 23/00 U.S. Cl. 164-81 3 Claims ABSTRACT OF THE DISCLOSUREThis invention pertains to lightweight, high strength elements forconstructional applications and, more particularly, to compositestructural articles comprising spacedapart face sheets or panels and animproved lightweight, high strength filler material in the form ofpreformed hollow spherical bodies or bubbles, The invention isespecially directed to methods for the manufacture of such hollowspherical bodies. The method comprises the steps of forming a sphericalbubble in a bath of molten material from which the sphere is to be madeand passing the formed bubble into a cooling medium overlying the bathto solidify the bubble.

This application is a divisional application of co-pending patentapplication Ser. No. 114,903 entitled Lightweight Structural Articles inthe name of Milton B. Vordahl, filed June 5, 1961, and now abandoned.

Many facets of those currently rapidly expanding, technological areasrelating to lightweight, high strength constructional applications, Suchas, for example, high speed aircraft, missiles, etc., have resulted inthe need for constructional materials exhibiting minimum weight togetherwith high strengths, over a wide range of temperatures. Many such newapplications have resulted in the development of metals and alloys, suchas iron, nickeland cobaltbase superalloys, the precipitation hardenablestainless steels and allied refractory metals and alloys, such as,titanium, columbium, molybdenum, beryllium, etc., having many propertiessuitable for use under rigorous environmental conditions, such as, hightemperatures, extreme temperature changes and corrosive atmospheres.Many of these metals, such as, titanium, beryllium and the like, havehighly desirable strength-to-weight ratios and, accordingly, have beensubjected to intensive development for those applications wherein highstrengths and low densities are prerequisites. However, many of thenewly available materials of construction, as well as others of the moreremote prior art, while possessing one or more desirable attributes,eg., high strength at elevated temperatures, corrosion resistance, etc.,are relatively dense materials. Moreover, even the lighter weightmaterials are frequently too heavy to use, in full sectional form, tothe extent necessary to obtain the desired strengths.

Consequently, the foregoing and other materials of construction havingbeen utilized, to a limited extent, in the form of laminates comprisingtwo or more relatively thin panels or sheets of the primary material ofconstruction, together with relatively lightweight cores. One example ofsuch composite structure is the so-called sandwich panel upon whichextensive developmental work is currently being expended, particularlyin those areas, such as aircraft and missile skin construction, whereinthe aforementioned properties of light weight together with highstrength are especially necessary. Such lightweight sandwich structures,as for aircraft use, are difficult to make in even the simplestembodiments, such as flat, spaced apart panels or skins havingcorrugated or honeycomb interior fill. Complex, curved shapes of suchconstruction are almost impossible to manufacture. In those instanceswherein such shapes are absolutely essential, the cost of the completedpanels ordinarily is one hundred times or more times the cost of theconstructional materials themselves. The ditliculty encountered in theconstruction of laminates of such complex shapes is occasioned, in part,by the necessity for the provision Ofcomplicated brazing fixturesrequired to maintain the close dimensional tolerances needed for suchstructures. Moreover, intimate contact of the spaced apart panels withthe interior fill is diicult to achieve throughout a large structure ofcomplex shape, requiring extensive and complicated machining operationsto tit the lill to the contour of the panels. Failure to achievesubstantially complete contact between fill and face panels results in adefective product which is apt to fail under applied stress.

Accordingly, it is an object of the present invention to provide alightweight, high strength sandwich structure susceptible of readymanufacture in either simple or complex shape, and of consequent lowcost.

It is a further object of the invention to provide a method of makingnew and improved high strength, low density, low cost sandwichstructures.

It is another object to provide a novel, lightweight, high strength fillfor laminated structural applications.

It is still another object to provide a method for making thin walled,hollow metal articles suitable for core fill for structural laminates.

In accordance with the above objectives, the invention provides, in apreferred embodiment thereof, a sandwich structure comprising relativelythin gauge metal sheets or panels held together and supported in apredetermined, spaced-apart relationship by a plurality of preformed,hollow metal spheres or bubbles. The spheres, constituting the interiorfill of the sandwich structure, are preferably provided with a film ofsuitable material such as a brazing alloy, which, upon heating of thefilled structure, melts and, upon subsequent cooling, solidifies toeffect adherence of the spheres one to another and to the confiningsheets or panels.

The foregoing and other objectives of the invention will be readilyapparent from an inspection of the following specification andaccompanying drawings herein.

FIG. l is an enlarged, cross sectional view of one form of a suitablehollow iiller sphere for use in manufacture of the sandwich structuresof the invention, the sphere being provided with a brazing coating;

FIG. 2 is an enlarged, cross sectional view of an alternative form of afiller sphere;

FIG. 3 is an enlarged plan View of the filler spheres, in a two-layerideal packing arrangement upon a bottom panel;

FIG. 4 is an enlarged, elevational View of a sandwich structure inaccordance with the invention, wherein the filler comprises athree-layer, ideal packing arrangement of ller spheres between twospaced-apart panels;

FIG. 5 is an isometric view of a structural article in accordance withthe invention comprising flat, spaced panels enclosing, and held inspaced-apart relationship by, a quantity of hollow filler spheres;

FIG. 6 is an enlarged, cross sectional, elevational view of a portion ofa sandwich structure in accordance with the invention, illustrative ofthe method of aflixing the filler spheres to each other and to the facepanels;

FIG. 7 is an enlarged, cross sectional View of a punch and die means forforming a first hemispheric portion of the hollow filler sphereillustrated in FIG. 2;

FIG. 8 is an enlarged, cross sectional, elevational view of a punch anddie means for forming a second hemispheric portion of the filler sphereof FIG. 2; and

FIG. 9 is an elevational schematic view, partly in cross section, of asuitable apparatus for producing hollow filler spheres in accordancewith the invention, i.e., forming bubbles of a suitable metal andsolidifying the same to form the spheres.

Referring now to the drawings, and more particularly to FIGS. 3, 4 and5, those drawings illustrate a simple at sandwich structure made inaccordance with the invention. Thus, the invention contemplates theprovision of relatively thin gauge side panels 11 of suitable material.Side panels 11 may comprise various metals, the nature of which isdependent upon the application intended for the finished article. Thus,for the especially important high strength, low density applicationsrequired in the aerospace industries, panels 11 may be fashioned from ahigh strength, high temperature metal or alloy, such as a precipitationhardenable stainless steel, titanium or alloys thereof, columbium,molybdenum, tungsten, zirconium, beryllium, etc. or alloys thereof.

As illustrated, the side panels 11 are spaced apart by a plurality ofhollow filler spheres 12. In order to realize the extraordinaryconstructional advantages of the product of the invention, the spheres12 are constructed of a suitable metal, the nature of which, as in thecase of the panels 11, depends upon the particular application for whichit is intended. Thus, the spheres 12 may themselves be made of the samehigh strength alloy as the panels 11. Alternatively, the spheres 12 maybe constructed of a metal or alloy diiferent from that of the panels 11.In the case of panels 11 made of a high strength, high temperaturealloy, the advantages of the invention may be realized even if thespheres 12 are made of less strong material, since the spherical shapeof the individual filler members is adapted to receive and to bearapplied high stresses, such as compressional stresses, with a minimum ofdeformation. Over-all strength and rigidity of the inventive compositestructures may be further enhanced by internally pressurizing thespheres 12 as with a pressurizing fluid such as air or an inert gas orother uid as will become apparent hereinbelow. Desirably, thepressurizing uid may be a lightweight gas, as helium or hydrogen, tofurther increase the strength-to-weight ratio of the pressurizedspheres.

The spheres 12 may be of any reasonable dimension, depending 4upon theconfiguration of and the properties desired for the composite article,so long as the diameter of the filler spheres is small relative to thedistance between the confining, spaced-apart panels; that is, thespheres are preferably small enough to accommodate a minimum of two, andpreferably three or more, complete layers of close packed configurationbetween the confining face sheets. This size relationship is importantin that so long as the spheres are small, they may be poured betweenpanels of preformed, complex shape and/or curvature and still eiectivelylill the volume between panels. Moreover, the stated minimum number oflayers promotes strength uniformity in the sandwich when subjected toapplied stresses of variable magnitude and direction. Thus, theinvention contemplates spheres ranging in diameter from a few tens ofmicrons Iup to several millimeters and larger depending on the minimumdistance between panels. The thickness of the walls of the individualspheres 12 may also vary widely in accordance with the propertiesdesired in the nished article and with the method utilized in themanufacture of the spheres.

FIGS. 3 and 4 show the spheres 12 deposited in an ideal packingarrangement. Such packing is conducive to minimization of excessivevoids and to maximization of strength and rigidity. It is to beunderstood, of course, that complete achievement of the illustratedideal packing arrangement is not necessary nor does it usually occur innormal practice. However, such packing arrangement of the spheres 12 isgenerally closely approximated in the practical construction of thearticles of the invention which, consequently exhibit, substantiallycompletely, the desirable attributes of that packing arrangement.

As illustrated in FIG. 5, the articles of the invention may additionallycomprise end or edge members or panels 13.

It is to be further understood, of course, that the invention is notlimited to but, indeed, explicitly contemplates shapes other than thesimple flat shapes illustrated in FIGS. 3-5. Thus, an enormous advantageolfered by the present invention is the provision of methods and meansfor producing complex structural shapes without the attendantdisadvantages and consequent high costs of the prior art. Thus, largeconstructional members of simple or compound curvature can be easily andquickly produced by simply lixing, in the desired spaced apartrelationship, the preformed sheets or panels, of any shape whatsoever,and the preformed spheres 12 then simply poured therebetween. There is,therefore, no necessity for extensive, time-consuming and costlyoperation upon an integral filler or spacer material in order to conformthe same precisely to the contours of the conning sheets or panels.

The hollow ller spheres 12 may be produced in any suitable manner,depending upon the size and the material of construction involved. Thus,for example, FIGS. l and 2 illustrate a hollow ller sphere suitable foruse in the invention. As will be noted from those drawings, the spheres12 may be formed in two, hemispheric sections, which sections maysubsequently be joined together to form the completed sphere. Thus, inthe case of the sphere shown in FIG. l, there is provided a firsthemispheric section 14 having an inwardly directed offset portion 16dem'ng a shoulder 17 and rim 18 adapted, respectively, Ifor the abutmentand resilient holding of a second hemispheric section 19. Similarly, inFIG. 2, a first hemispheric section, designated generally by the numeral21, is provided with an outwardly directed offset portion 22 defining ashoulder 23 and a flange 24 adapted, respectively, to abut and toresiliently hold a second hemispheric section, designated generally bythe numeral 26.

A hollow iller sphere, as illustrated in FIG. 2, may be made, forexample, by punch and die means as illustrated in FIGS. 7 and 8. Thus,in FIG. 7 there is provided a first die block 27 provided with aplurality of die cavities 28 for reception of lirst punches 29. By useof such apparatus, lirst hemispheres 21 may be easily and quickly madein large numbers simply by placing a flat sheet structure 31 upon thesurface of die block 27 and actuating the punches 29. Similarly, secondhemispheres 26 may be rapidly made in quantity by means of a die block32 having a plurality of die cavities 33 for cooperation with acommensurate number of second punch members 34.

The spheres 12, of course, serve not only to space the panel membersapart but serve also to fixedly hold the same in a predeterminedposition. This is accomplished, for example, by providing each of thespheres 12 with an overlying layer 36 (FIG. l) of a suitable bondingmaterial such as a brazing alloy. The bonding medium may fbe applied tothe spheres 12 by any suitable means, as by dipping the spheres inmolten bonding medium, spraying, etc. Thereby, after the preformedhollow spheres are poured into place lbetween the confining panels, theentire assembly is heated to a temperature at which the bonding mediumbecomes sufliciently Huid to How about the spheres and to collect and beheld by surface tension forces, in large part, at the points of contactbe tween the individual spheres and between the spheres and theconfining panel sheets. Upon subsequent cooling and solidiication of thebonding medium, the component parts of the assembly are converted intoan integrally bonded, unied whole. The bonding of the composite parts ofthe assembly iby the bonding medium is illus trated in FIG. 6y whereinthe bonds between spheres 12 are illustrated at 37 and those lbetweenspheres and panels 11 are illustarted at 38.

As an alternative to the mechanical formation of the hollow fillerspheres 12 illustrated iri FIGS. 7 and 8, the spheres may be formed inthe manner illustrated in FIG. 9. Thus, in that drawing there :isillustrated an apparatus comprising a relatively elongated verticalcolumnar vessel 39 which may be provided with suitable heating means ina lower portion thereof, as heating coils 41. Heat loss from the vessel39 may be minimized by providing an insulating cover 42 thereabout. Alower portion of the vessel 39 contains a first uid medium 43 comprisinga metal from which it is desired to form the spheres 12. Floating on topof the first fluid medium 43 is a lighter, second uid medium 44.Substantially constant amounts of the two fluid media may be maintainedwithin the vessel 39 by providing reservoirs 46' and 47, connected tothe appropriate portions of the vessel 39 by inlets 48 and 49respectively, the inlet lines being provided, respectively, with controlvalves 45 and 50. Pumps 51 and 52 are pro vided to supply fluid fromreservoirs 46 and 47, respective ly, to the vessel 39. There is alsoprovided adjacent the bottom of the vessel 39 an inlet line 53 which isconnected to a vertically extending nozzle 54. A controller valve 56 isprovided in the inlet line 53 and is connected to a pressurizing pump 57which, in turn is connected, by supply line 58, to a source ofpressurizing uid (not shown). A discharge chute 59 is provided adjacentthe top of the vessel 39 and may have an exit extremity there ofadjacent a suitable transferral means, such as conveyor 61.

In operation of the apparatus illustrated in PIG. 9, a pressurizing gas,the composition of which is depend ent upon the composition of medium43, is pumped into the vessel 39 through nozzle 54 whereupon a bubble 62is formed within the medium 43 and rises therethrough and passes intothe lighter, immiscible medium 44, the upper portions of which aremaintained at a temperature below the freezing point of medium 43whereby the film of medium 43, comprising the skin of the bubble 62,freezes to form a solidified, hollow sphere 12. The spheres 12 oat andcollect upon the surface of medium 44 and are removed therefrom, throughthe chute 59, to the conveyor 61 by means of which they may betransferred to the location of any subsequent processing to which it maybe desired to subject them.

The particular process features utilized in connection with theoperation of the apparatus .in FIG. 9 Will depend upon many factors,among the foremost of which are, of course, the nature of thebubble-forming medium 43. As an example of spheres which may be producedin the aforesaid manner, molten aluminum is provided as the medium 43and over the aluminum is floated a molten mixture of halide or othersubstantially unreactive salts having a melting point below that ofaluminum as, for example, a low melting mixture, such as a neareutectic, of certain salts, e.g. chlorides of sodium, potassium,lithium, calcium, etc. A suitable temperature gradient is set up in thesalt layer as, for example, by the provision of suitable cooling means(now shown) adjacent the top of the vessel 39. A small jet of air isthen blown into the molten aluminum through nozzle 54 thereby formingbubbles 62 having an inner, continuous aluminum oxide film wetexteriorly thereof with molten aluminum. As the bubble rises and passesacross the aluminum-salt interface into the salt layer 44 the aluminumfilm solidifies, providing an air filled aluminum bubble.

By way of further example, the lower layer 43 may comprise molten zinccontaining titanium to the limit of its solubility and whichadditionally contains iinely divided, suspended particles of titanium ortitanium alloy. The upper layer 44 again comprises a molten mixture ofhalide salts having a melting point below that of the metallic layer 43.Bubbles are then formed in the metal slurry by means of a jet of inertgas, argon or helium, containing a small partial pressure of a gas suchas air, carbon monoxide or a suitable carbonyl which will react with theslurry to form a stabilizing skin. After solidification by risingthrough the salt layer 44 and removal from the vessel 39, the metalbubbles are heated in a partial vacuum to remove the zinc and to Sinterthe titanium particles thereby producing a continuous titanium sphere.

In a still further example, the lower layer 43- may comprise tin,aluminum or zinc containing dissolved sodium and the upper layer 44 maycomprise a molten halide mixture containing a lower chloride of titaniumor other metal of which it is desired to form the bubble and which isreducible by the sodium. In such case, as the sodiumcontaining bubblerises through the halide layer, there is formed on the surface thereof,by reduction of the metal chloride, a thin film of the correspondingmetal.

The advantages of the invention are not restricted to the specifiedapparatus mentioned, but may lbe utilized in a wide variety of other,less rigorous applications. Thus, the low cost and inherentlyadvantageous properties, such as great strength, low density,substantial absence of directionality, vis-a-vis applied stress,admirably suit these novel sandwich structures to architecturalconstruction and the like. Thus, in addition to possessing high loadbearing capacity, the inventive structures may inherently possess,depending upon the materials of construction, a high degree ofsoundproofng and/or insulating properties.

As noted, the articles of the invention exhibit substantially uniformresistance to deformation of various applied stresses. Thus, theresistance to shear stress applied at various angles to the plane of thesuface panels is substantially as great as the resistance to compressivestress applied normally to the panels. The inventive articles have highresistance to tensile stress also. These prop erties distinguish theinventive articles from prior art sandwich structures with theirvariable directionality. Thus, corrugated sandwich filler is Weak inbending with the bend axis parallel to the corrugations. Honeycombfiller is weak in resistance to shearing movements of the face sheets.The sandwich structure of this invention, on the other hand, exhibits ahigh degree of uniformity of strength properties. Thus, although theinventive structures are not as strong as corrugated structures in thebest direction of the latter, they are much stronger than a corrugatedstructure in its worst direction. My new structures are alsoexceptionally strong in resistance to applied shear stress-incontradistinction to honeycomb structures. My novel structures are alsoto be distinguished from sandwich structures comprising a foamed metalfiller. In the latter type of articles, the interstices between the foambubbles are filled with metal, thereby contributing significantly to theweight of the sandwich and, consequently, making such structuresunsuitable for those applications requiring highest strength-to-weightratios. Moreover, bubble wall thickness is uncontrollable in thefoamed-type material, with a concomitant decrease in uniformity ofproperties. Additionally, use of the foamed metal procedure severelylimits the choice of metals which can be utilized. The herein describedinventive structures avoid those last-mentioned disadvantages.

By way of further example of specific structural articles in accordancewith the invention, I contemplate sandwich structures comprising facesheets of a high strength, high temperature, low density metal, astitanium or its alloys and a filler of titanium spheres, the wholejoined by a bonding medium comprising magnesiumin accordance with theteachings of my co-pending United States patent application Ser. No.114,903, now abat-1- doned. Further examples comprise face sheets oftitanium, aluminum or stainless steel face sheets and a filler ofaluminum spheres, the whole joined by an epoxy resin bonding medium. Ialso specifically contemplate face sheets of titanium, titanium fillerspheres and a low melting point titanium base brazing alloy, the latteras described and claimed (3% beryllium, balance titanium) in co-pendingapplication Ser. No. 770,608 inthe name of Howard B. Bomberger, Jr. Afurther example of particular, useful embodiment of the inventioncomprises face sheets of titanium, titanium alloy or stainless steel andller spheres of titanium, stainless steel or aluminum, wherein bondingis elected by thermosetting metal alloy pastes, as, for example, thosedescribed in my United States Patent No. 2,837,425.

What is claimed is:

1. A method of making a thin-walled hollow metal sphere, comprisingforming a gas spherical bubble in a bath of molten, sphere-formingmetal, said bubble being enclosed by a partially solidied layer of saidsphereforming metal, passing the bubble into a layer of a liquid coolingmedium overlying the bath, and cooling the bubble within the liquidcooling medium to form a solid, Wallorming skin of sphere-forming metalthereabout.

2. A method of making a thin-walled, hollow metal sphere, comprisingforming a molten metal bath, injecting gas into said bath below thesurface thereof to form a bubble therein, passing Vthe bubble into a'cooling zone comprising a molten salt, oflesser density and lowermelting point than the metal, overlying the metal bath, cooling anupperextremity of the cooling zone below the freezing point of themetal, and passing the bubble into said cooling zone `whereby a metalwall thereof freezes to form the desired hollow sphere.

3. A method of making a thin-walled, hollow aluminum sphere, comprisingforming a moltenf aluminum bath,

` form a self-supporting, sphere wall of aluminum.

References Cited UNITED STATES PATENTS 852,396 4/1907 Pease. 3,013,31112/1961 Meissner 18-57 3,230,056 1/1966 Arant et al. 164-267 X 2,136,096ll/l938 Benner et al 65-'-21 X 1,236,606 8/1917 Sanford 65--2291,871,792 8/1932 Horseld 65-21 X '2,583,452 l/l952 Watts et al. 65-21 X2,624,069 1/1953 Fisher 18-5 X J. SPENCER ovERHoLsER, Primary ExaminerV. K. RISING,'Assistant Examiner

